Most studies project that second-generation liquid biofuels from perennial crops and woody and agricultural residues could dramatically reduce life cycle greenhouse gas emissions relative to petroleum fuels. Some options hold the potential for net emission reductions that exceed 100 percent – meaning that more carbon would be sequestered during the production process than would be emitted as carbon dioxide during its life cycle – if fertilizer inputs are minimized and biomass or other renewable sources are used for process energy (see Worldwatch Institute, 2007).
Studies suggest that use of bioethanol produced from maize represents only a slight improvement in fossil fuel use efficiency over direct use of petroleum, while bioethanol produced from wood can improve energy efficiency by up to four times (NRDC, 2006). Estimates put greenhouse gas emissions for biomass- based second-generation fuels at 75 to 85 percent below those of petroleum motor fuels, because of less-intensive farming and the assumption that the unfermentable portion of the plant is used as the processing fuel (Global Insight, 2007). Thus, if technological developments make it more efficient and at least as economical to produce liquid biofuels from cellulosic material rather than from food crops, the result would be reduced competition with food production, an increase in energy efficiency and improved overall energy balance. This could result in incentives to expand forest plantations.
Compared to gasoline or diesel, greenhouse gas emissions are lowest for biomass to liquid processes (i.e. gasification/pyrolysis processes that can utilize the whole plant). Sugar cane is similarly placed and cellulosic ethanol reduces emissions by over 75 percent. Ethanol sourced from wheat returns poor emission reductions unless the wheat straw is also used in CHP processes (Figure 15).
Sugar cane is the most economically attractive agricultural feedstock for liquid biofuel, while maize and other cereal and oilseed crops from the Northern Hemisphere are less competitive under market conditions (Figure 16). While the present costs of producing ethanol from cellulose are higher than those from cereal feedstocks, the potential for reducing production costs in the future appears to be much greater for cellulosic ethanol. By 2030 parity with ethanol from sugar cane may be possible (IEA, 2006).
The development of an economically viable process for producing cellulosic liquid biofuels could lead to the widespread use of forest biomass in the transport sector. As most of the growth in demand for liquid biofuel is expected in developed countries, the scope for trade is the main factor affecting development plans in the majority of developing countries.
Feedstocks and processes that do not produce significant net energy gains are less likely to be supported by markets, although it is possible that other objectives may perpetuate their production (Wolf, 2007). It is unlikely that crops grown specifically for the production of cellulosic biofuels will be developed in significant quantities as technology gains and bioethanol prices are unlikely to favour production over alternative crops. Similarly, it is not expected that stand-alone second-generation bioethanol and biodiesel plants will be profitable in the coming decades (Global Insight, 2007). The competitiveness of different feedstocks is related to the net energy efficiency associated with production and processing of different crops (Box 5).
BOX 5: Energy efficiency and bioenergy production
Energy consumption in bioenergy production is important for two reasons. Firstly, to be sustainable, the amount of energy gained in growing and utilizing an energy crop must exceed that used in producing the crop. Secondly, the types of fuel used for the energy inputs and their greenhouse gas emissions must be taken into account where climate change goals are targeted through bioenergy use.
Energy use is dependent on a number of factors. Agriculture requires energy inputs at many different stages, including for powering farm machinery, irrigation and water management and transporting products. Large amounts of energy are also consumed in activities associated with agriculture, such as fertilizer and pesticide manufacturing and processing, and distribution of agricultural products. This is especially the case in modern high-input farming systems.
Agriculture in industrialized countries is generally much more energy intensive than in developing countries, although as they move to more advanced cultivation practices, energy inputs tend to increase. In many cases, energy inputs are likely to be from fossil fuels. For this reason, the production and use of bioenergy resources only marginally reduces carbon emissions in comparison with fossil fuel use.
The major advantage of forests and trees as a source of biomass is their lower energy inputs and their ability to grow on sites with lower fertility than those required for agriculture. There are, however, major constraints to capitalizing on these advantages including the timely emergence of second-generation technologies, the future supply of wood and the infrastructure necessary for economic viability (Perley, 2008).